This invention relates to an oxide thin film for bolometer-type uncooled infrared detector with a small temperature resolution capability.
A bolometer uses a thermal variation of resistance in metal or semiconductor thin film that is thermally isolated from a substrate material. Japanese patent application laid-open No. 5-206526 (1993) discloses a technique that n-or p-type doped amorphous silicon (a-Si) is used as a bolometer material. Also, U.S. Pat. No. 5,300,915 discloses a technique that an alloy of nickel-iron is used as a bolometer material.
Characteristics required for bolometer material are a temperature coefficient of resistance (TCR) and a value of resistance. In general, a high resistivity of bolometer material is not suitable since the Johnson noise increases with an increase in resistivity. Also, a low resistivity is not suitable since the difference between the resistivity of wiring except the bolometer and the resistivity of bolometer material becomes small. Thus, it is desirable that the resistivity of bolometer material is about 5 kxcexa9 to 100 kxcexa9 at room temperature. In other words, when a formable thickness of bolometer thin film is 0.05 to 1 xcexcm, it is desirable that a resistivity required for bolometer material is about 0.025 xcexa9cm to 10 xcexa9cm.
The temperature resolution capability (NETD: noise equivalent temperature difference) of infrared sensor is inversely proportional to the absolute value of TCR of bolometer material. Therefore, by using a bolometer material with a big absolute value of TCR, an infrared sensor with a small NETD can be obtained. However, as described in prior arts, although n- or p-type doped amorphous silicon has a TCR as high as 3 to 3.5%/K, its resistivity must exceed 1xc3x97103 xcexa9cm.
Although Japanese patent application laid-open No. 5-206526 does not describe, the resistivity of nickel-iron alloy is as small as 40 to 70 xcexcxcexa9cm (referenced from The Metal Handbook) . Therefore, it is assumed that its absolute value of TCR is similar to that of another metal with a like resistivity and is not more than 0.5%/K. Thus, such a material is not suitable for bolometer material used for an infrared sensor with a small temperature resolution capability.
As a solution to the above problem, U.S. Pat. No. 5,286,976 discloses an infrared sensor that vanadium oxide or titanium oxide is used as a bolometer material. Although this prior art does not describe about the characteristic of these bolometer materials, for example, Tsuda, xe2x80x9cConductive Oxidexe2x80x9d, Shokabo Shuppan, p.24 exhibits graphs illustrating the thermal variation of resistivity about vanadium oxide (V2O3 etc.) and titanium oxide (Ti2O3 etc.). Although these graphs do not relate to thin films, it is assumed that a suitable resistivity and a big TCR can be obtained by setting a suitable temperature. However, near at room temperature, the resistivity and TCR are not always suitable.
Japanese patent application laid-open No. 9-257565 (1997) discloses a technique that applies vanadium oxide, which incurs no phase transition from below the freezing point to over 100xc2x0 C., to bolometer thin film. This prior art describes that its application to infrared sensor is advantageous in that the absolute value of TCR exceeds 1%/K and no variation in volume occurs. However, the absolute value of a TCR obtainable is limited.
In many of oxides with conductivity, there exists a temperature region that a big TCR with phase transition is exhibited at a certain temperature. A technique that shifts the temperature region with a big TCR to room temperature by doping a suitable material is reported. For example, S. Koide et al., xe2x80x9cPreparation of Doped VO2 Single Crystals and Their Electrical Propertiesxe2x80x9d, Applied Physics, Vol.37, No.9, pp.815-820 (1968) reports that transition temperature is shifted by doping titanium (Ti), niobium (Nb), silicon (Si), germanium (Ge) or tin (Sn) into single crystal of vanadium oxide (VO2)
Also, C. B. Greenberg, xe2x80x9cUndoped and Doped VO2 Films Grown from VO(OC3H7)3xe2x80x9d, Thin Solid Films, 110(1983)73-82 reports that VO2 thin films doped with tungsten, molybdenum and niobium are grown on glass by CVD, thereby the transition temperature can be shifted from near 70xc2x0 C. for undoped to a lower temperature. In this report, in case of W 1.4 mol % doping, the transition temperature is about 40xc2x0 C., the resistivity is 1.1 xcexa9cm, and TCR =xe2x88x925.5%/K. Also, in case of Mo 1.8 mol % doping, the transition temperature is about 50xc2x0 C., the resistivity is 0.3 xcexa9cm, and TCR =xe2x88x929%/K. It is reported that these materials are used in applications to a critical temperature thermistor or near-IR switching element. In this regard, it may be assumed that to apply vanadium oxide doped with another element to thin film for bolometer facilitates, in calculation, to give a reduced noise and to give a temperature resolution capability smaller than that in prior art, but there is no literature reporting such application. Measuring the temperature dependency of resistance in Mo-doped vanadium oxide thin film, there is a difference between resistivities in the rising and falling of temperature. In other words, the thermal hysteresis of resistivity is observed. This is an unsuitable characteristic to bolometer material for infrared sensor.
T. E. Phillips et al., Materials Research Bulletin (1987) p.1113 reports that Fe-doped VO2 thin film is formed on a glass substrate by reactive sputtering, and its electrical resistance and transition temperature observed from resistivity are tested. In this report, the resistivity is 1 to 10 xcexa9cm near (35xc2x0 C.) at room temperature. Thus, the resistivity is too high for bolometer thin film. In fact, an application to bolometer thin film for infrared sensor is not suggested in this report.
Accordingly, it is an object of the invention to provide an oxide thin film for bolometer that offers a low resistivity and a large TCR value.
It is a further object of the invention to provide an infrared sensor that offers a finer temperature resolution capability (NETD).
According to the invention, an oxide thin film for bolometer, comprises:
a vanadium oxide represented by VOx, where x satisfies 1.5xe2x89xa6xxe2x89xa62.0,
wherein part of vanadium in the vanadium oxide is substituted by metal ion M, where the metal ion M is composed of at least one of chromium (Cr), aluminum (Al), iron (Fe), manganese (Mn), niobium (Nb), tantalum (Ta) and titanium (Ti).
According to another aspect of the invention, an infrared sensor, comprises:
a bolometer;
wherein the bolometer is of oxide thin film that comprises a vanadium oxide represented by VOx, where x satisfies 1.5xe2x89xa6xxe2x89xa62.0,
wherein part of vanadium in the vanadium oxide is substituted by metal ion M, where the metal ion M is composed of at least one of chromium (Cr), aluminum (Al), iron (Fe), manganese (Mn), niobium (Nb), tantalum (Ta) and titanium (Ti).